The past decades have seen a considerable rise in the deployment of mobile telephony. After the slow start of analog standards like AMPS, Nordic Mobile Telephone (NMT) and the Total Access Communication System (TACS), mobile telephony has recently become quite popular in the consumer markets with products employing advanced digital standards like GSM and D-AMPS. In addition to other developments in mobile phone features, like smaller size and longer battery life, much progress has been made at the network side as well, particularly, in frequency reuse schemes to avoid co-channel interference between adjacent cells. Increasingly, dense cell reuse plans have been complemented with hierarchical cell structures, where macrocells cover entire districts, microcells cover smaller parts like streets, and picocells cover very small areas the size of a few rooms. Important for the hierarchical cell structure is that all the base stations deployed (ranging from macro to pico base stations) are part of the same public land mobile network (PLMN).
In order to avoid co-channel interference between different radio links, a structured channel allocation scheme is applied. Within a cell, the cellular base station makes sure that connections to different mobile stations are carried over different carrier frequencies and/or different timeslots. In order to suppress co-channel interference from surrounding cells, a frequency reuse scheme is applied in which the cellular operator plans the frequencies such that adjacent cells do not use the same frequency set. For example, in conventional AMPS systems, a 7-site/21-sector reuse methodology (7/21) is applied, which means that in a cluster of 21 sectors, all of the applied frequencies therein are unique. In modern cellular digital systems, however, more dense reuse schemes are applied, like a 4/12 or even a 3/9. In more advanced cellular systems, slow frequency hopping (FH) traffic channels have been introduced. For example, in GSM, the traffic channel is allocated a fixed slot, but in each transmission frame a different carrier frequency corresponding to a particular hopping sequence is used. Traffic channels belonging to a single base station use orthogonal hopping sequences. This means that there is never a collision between connections controlled by the same base. However, between different base stations, random hopping may be applied and collisions may occur.
Recently, private networks for residential and business areas have been developed, which although using the same air-interface and the same spectrum as the cellular system, are not integrated with the overlaying public cellular network. In this sense, these private systems cannot be considered as micro or pico networks since there is no direct communication between these private systems and the cellular system. For example, for residential usage, private base stations can be used as described in either U.S. Pat. Nos. 5,428,668 or 5,526,402 which only connect to a Public Switching Telephone Network (PSTN).
If, however, such a private radio communication system is placed into an area covered by the cellular system with which the private system has to share frequencies, a problem arises since the private base stations are not coordinated with the cellular network. Therefore, they are not incorporated into the frequency reuse plan of the cellular network. Moreover, they are not coordinated among themselves. Accordingly, a method is needed which both prevents the private radio system from interfering with the overlaying cellular system, and which also prevents interference among different private radio systems covering the same area.
A recent patent application of the assignee, of which the present inventor is a co-inventor, entitled "Methods and Systems for Allocating a Cellular Communications Channel for Communication between a Cellular Terminal and a Telephone Base Station Using Received Signal Strength Measurements," Ser. No. 08,517,710, filed Aug. 21, 1995, describes a method in which downlink measurements on control channels performed within the mobile station are sent to a cellular-operator-controlled server in the fixed network, which subsequently allocates to the private telephone base station a set of traffic channels corresponding to the control channel with the lowest received signal strength. This method, however, requires a correlation between the cellular traffic channels and the cellular control channels, a correlation function which must be present in the database of the server. In addition, replanning a cellular network again requires the interaction of the operator's server.
A more autonomous method was described in the aforementioned U.S. patent application Ser. No. 08/704,846, entitled "Method and System for Autonomously Allocating a Cellular Communications between a Cellular Terminal and a Telephone Base Station", in which a combination of a slow adaptive frequency allocation and a fast-responding dynamic channel selection was described. First, a set of frequencies was derived that minimally interfered with the cellular system. Then from this set the best channel at that point in time was selected where a "channel" was a frequency and a timeslot.
Another technique used to facilitate the co-existence of uncoordinated systems in a particular area is frequency hopping, a general technique which spreads the interference among a number of users. Because the aforementioned private systems are neither coordinated with each other nor with an overlapping cellular system, only random FH may be applied. Sporadic collisions are overcome by the combination of frame interleaving and Frequency Error Correction (FEC), as is understood in the art. However, if the number of collisions increases, the system breaks down since the interleaving and FEC can only cope with a small amount of errors, i.e., a low collision rate.
To provide a low collision rate in random FH systems, either the number of frequencies to hop over must be large or the traffic load on the frequencies must be small. Both conditions are hard to fulfill in private base stations sharing the frequency spectrum with an overlapping cellular system. First, due to hardware limitations, current cellular terminals can only hop over frequencies and not over timeslots within a transmission frame. Further, in order to avoid interference to the cellular system (especially if there is a high density of private base stations) the private radio systems should not use the carrier frequencies that are presently in use by nearby cellular base stations. This leaves the number of carrier frequencies to hop over in the private radio system rather limited, with rather great danger for interference (collisions) between different private radio systems which are not coordinated but which may still have considerable overlapping coverage areas. However, FH in uncoordinated systems is nonetheless advantageous because FH in general provides interference spreading and also reduces interference to unknown or unexpected users sharing the same spectrum. In addition, FH combats multipath fading provided the frequencies to hop over span a sufficiently wide spectrum, as is understood in the art.
It is therefore an object of the present invention to provide a method and system in which carrier allocation occurs in the private base station automatically to avoid interference situations.
It is a further object of the present invention that the method and system be adaptive, in that if the cellular network is replanned, the private network will automatically replan as well in order to avoid interference conditions.
It is also an object of the present invention that the method and system respond automatically upon encountering interference from other nearby private base stations and provide interference spreading among any users sharing one or more of the frequencies.
It is another object of the present invention to provide a method and system which allow a private radio system within a larger cellular network to autonomously determine those frequencies and frequency/timeslot combinations it can use with minimal disturbance to and from the overlaying cellular system.
It is still another object of the present invention that the method and system in the private radio communication system automatically select a frequency/timeslot hop set that prevents interference with the overlaying cellular system as well as preventing interference with other, nearby private radio systems.